In a large bowl, beat butter and maple sugar with an electric mixer on medium speed until creamy and smooth
Beat in yolk until fully incorporated
In a medium bowl, whisk flour, baking powder and salt
Add flour mixture to butter mixture and beat for about 20 seconds, until a smooth dough comes together
Pat dough on the counter to form a 1x2x7 inch brick
Wrap in plastic and refrigerate dough for at least 4 hours, until completely chilled
Preheat oven to 375 degrees F
Use a sharp knife to cut half the dough into 1/4 inch slices
Refrigerate remaining dough
Arrange slices on the baking sheet, spacing them generously
Sprinkle with turbinado sugar, if using, per cookie
Bake for 8-10 minutes, until lightly browned
Remove from the oven and cool on baking sheet for 5 minutes before transferring cookies to a cooling rack to cool completely
Repeat with remaining dough